During the past decade flipchip technology has emerged as a popular alternative to wire bonding for interconnecting semiconductor devices such as integrated circuit (IC) dies to substrates such as printed circuit boards, carrier substrates, interposers and other dies.
“Flipchip,” is also known as “controlled collapse chip connection” or its acronym, “C4.” With flipchip technology, solder bumps are attached to electrical contact pads on one face of a die/chip. The flipchip dies are usually processed at the wafer level, i.e., while multiple identical dies are still part of a large “wafer.” Solder bumps are deposited by an electroplating process on chip pads on the top side of the wafer and then reflowed to form a bump. The water is sometimes “singulated” or “diced” (cut up into separate dies) at this point to provide a number of separate flipchip dies each having solder bumps on the top face surface. the chips may then be “flipped” over to connect the solder bumps to matching contact pads on the top surface of a substrate such as a printed circuit board or carrier substrate on which the flipchip is mounted. Solder bump attachment is usually provided by reflow heating.
As IC dies have become more complex, the number of solder bumps on flipchips have increased dramatically. Whereas in the past the solder bumps were usually provided by relatively large round solder bumps attached to the chip contact pads, more recently copper pillars (“CuP's”) have been used in place of the solder bumps. A CuP is an elongated copper post member that is attached at one end to a contact pad on the flipchip die. The CuP extends outwardly from the die in a direction perpendicular to the face of the die. Each CuP has a generally bullet or hemisphere shaped solder piece attached to its distal end. The CuP's are bonded by this solder piece to corresponding contact pads on a substrate as by reflow heating. CuP's are capable of being positioned much more densely, i.e., at a “higher pitch,” than conventional solder bumps. For example, a typical pitch for a flipchip solder bump array is 150 μm, whereas a typical pitch for a flipchip CuP array is 40 μm. One manner of facilitating connection of a substrate to a die having such high CuP density is to provide bond “traces” (also referred to as “fingers”), rather than conventional contact pads, on the substrate to which the flipchip is to be mounted. The traces are elongated contact pads that may be positioned in close parallel relationship, traditionally without any insulating material between them.